Giyuu Nii-san
by TomiokaGiyuuChild
Summary: Demon Slayer AU. After the tragic death of his sister, Tomioka Tsutako, Tomioka Giyuu thought that he didn't have family no more until Tanjirou just introduced him with a boy. Giyuu will now take care of his little brother. Big brother!Giyuu AU.
1. Mysterious Little Brother

**Giyuu Nii-San**

**This is another AU. Tomioka Giyuu is an older brother of a 10-year-old brother. The disclaimer is that I do not at all own the Demon Slayer series, their proprietary is Koyoharu Gotoge. I only own this plot and my OCs, Tomioka Kyuu and Akira.**

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"Tanjirou! What is going on in there?" Zenitsu cried out to his friend, followed by Inosuke. It was dark and already night, the three Demon Slayers got in a battle with a not very high demon, it was just a rookie. But when Tanjirou fended it off, something got his attention. A small boy, with spikey dark hair was cowering in there. Tanjirou believed he looked like the Water Hashira, Tomioka Giyuu, his mentor and the Hashira he had a high respect for.

"I found a boy!" Tanjirou replied, crouching down to said boy. "Excuse me, are you all right there?"

The boy nodded, but still was shaking, he had been chased by the demon for long now. Tanjirou still thought, the resemblance with Giyuu this boy had was undeniable.

"What's your name?"

"I'm...I'm Tomioka Kyuu..."

Tanjirou's eyes widened and his jaw dropped a bit at the revelation the boy just told him. A Tomioka? Giyuu's son? No, impossible, Giyuu was like 19 and didn't have a mate, and wasn't even interested in building a family of his own. But he knew he was quite interested with that nice, redheaded woman, Akira. That was undeniable, the only person besides Tanjirou, that the Hashira showed a soft personality.

"Kyuu-kun... Your last name is Tomioka. Do you by any chance know, someone by the name Tomioka Giyuu?" Tanjirou inquired, wanting to know what was the relation he had with Giyuu.

Kyuu slowly nodded his head. "He's my Nii-san. I never met him, but I knew I had a Nii-san and a Nee-san. I was hidden for the major part of my life... I never met Giyuu-Nii-san, but I could meet Tsutako-Nee-san. I've been trying search for him, maybe he's still alive."

Tanjirou placed a hand on Kyuu's shoulder and smiled softly at him. "He is very much alive, Kyuu-kun. I know him and respect him dearly."

Kyuu's blue eyes widened. "Can you please take me to him? I want to meet him finally!"

Tanjirou nodded and straightened. "Of course. But it seems that you sprained your ankle when you were running from that demon, you'll need some medical treatment for that. I can carry you." He kindly said, turning and crouching down so Kyuu could climb up. The boy got helped by Zenitsu who helped her on his back. Kyuu shakily held on, he looked so frail and sickly and pale, which worried Tanjirou and Zenitsu, Inosuke wasn't worried at all. Instead quite annoyed with the two of them taking in a random child.

"Tanjirou are you taking him to the medics after showing him to Tomioka-san?" Zenitsu asked, worriedly for the child's health.

"Of course. He seems pretty pale to me." Tanjirou stood up and Kyuu gripped him weakly, letting out a cough.

"It's a waste of time!" Inosuke as usal complained. "Why don't you just leave that brat here!" The boar guy, who ironically had a feminine face growled out, vapor coming out his nose. Kyuu whimpered at that.

Tanjirou scowled at Inosuke. "Shut up. You can easily walk ahead of us. But I will help this poor boy." He said bluntly and sped up ahead of Inosuke with Zenitsu along side. Inosuke was left shouting as he usually did, not happy he was left out.

Meanwhile, Giyuu was off walking, after all the Hashira's finished the meeting with the Master. When he heard Tanjirou's voice calling out for him and he slowly turned to the direction. It surprised him to see, as Tanjirou got closer the boy on his back, who was that child? Did Tanjirou save him?

"Tomioka-san! Have a moment to spare?" Tanjirou asked and stopped before the Hashira.

"Of course. Go ahead, Tanjirou. What is it?" Giyuu asked calmly.

"Giyuu-Nii-san?" Kyuu spoke before Tanjirou could and Giyuu widened his eyes big.

"Excuse me? Do you know me?"

"Tomioka-san, he is your little brother, according to his explanation. A demon was after him and I saved him. He said he knew you were his brother, but never met you before, but he met Tsutako-san." Tanjirou explained and Giyuu was in shock, lots of feelings mowed his mind. Why wasn't he introduced with this boy who apparently was his little brother. Why did Tsutako never told him they had a younger brother. Giyuu shook his head and approached closely.

"What's your name?"

"T-Tomioka K-Kyuu..." Kyuu answered his older brother with a quivering voice. He quivered and his blue eyes formed tears. Giyuu immediately softened his sharp gaze.

"No need to cry. I merely asked you who were you. But since now I found out my younger brother." Giyuu picked him up from Tanjirou's back and gently held him. "It's my responsibility to care for you." He looked at Tanjirou. "Thank you, Tanjirou. I shall take over from now." He disappeared out of Tanjirou's sigt and the other younger boy nodded.

"No problem, see that he gets better, Tomioka-san!"

Giyuu hurried to the Butterfly State to ask his fellow Hashira, Shinobu. As much as it annoyed him, but he didn't know where else to go.

"Kocho. Something happened. Apparently I'm not the only Tomioka here. Tanjirou found my little brother. He's bad, sprained ankle and unknown disease." The Water Hashira told the Insect Hashira urgently, or as his monotonous tone sounded. Kyuu gripped Giyuu, whimpering, he had been tearing up silently all the while, both fearful and happy his brother accepted him quick, he was thinking that Giyuu wouldn't see him as his brother, but apparently, he was way more caring.

"Ara? That's surprising. But yes, over here." Shinobu led the way to an empty room as Giyuu held his brother close and soothingly, walking hurriedly after Shinobu.

"It's going to be all right, Kyuu. I'm here."

Shinobu thought there wouldn't be the day Giyuu would sound so caring and compassive, given the man was simply an unreachable stone of no-emotions. But turned out Giyuu was very emotional, but only with a few people. She pat the bed for the Hashira to lay his brother down. Kyuu whimpering and started crying scared, Giyuu frowned and layed a hand to the kid's cheek and rubbed softly.

"It will be okay." He soothed as Shinobu took a syringe and gently injected the boy's ankle for it heal quickly. Kyuu bawled his eyes out when at the feeling of the sharp needle being administered. Giyuu patted his cheek softly, comforting the crying child. Shinobu started bandaging the ankle to up the leg gently. Then she left to prepare a remedy for whatever diseased this boy had.

"It's okay, it's all right. You'll get well soon, Kyuu-kun." Giyuu soothed him and wiped away the tears with his sleeve. He was worried for his brother's health, just recently found out he had family somewhere, he was going to protect Kyuu with all his life.

Kyuu sniffled and nodded, the needle did hurt but it was effective the pain in his ankle was slowly going away. Shinobu came back with the remedy.

"I'll need you to drink this for me, Kyuu." Giyuu held the glass close to the boy's lips, helping him drink. The boy drank without issue and his brother smile a little. "Good boy." He praised and patted his head.

"He'll need to rest for a bit here, I'm sure the next day he'll be like new." Shinobu assured and Giyuu nodded, movie a chair to sit next to his brother.

Kyuu felt secure now, Giyuu was nice and caring with him, he didn't feel intimidated at all, in fact very loved. His eyelids started to get heavy and he fell right into a deep slumber.

Giyuu kept rubbing his cheek, a very soft smile plastered in his face and Shinobu thought that was a big rarity, she left the Tomioka brother's to themselves, she would come back if she was needed, but for now the situation was under control.

"Shinppai-shinaide, Otōto." Giyuu whispered. "I'll take care of you, got my promise."

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**This AU is a different time from the time in New Hope, New Beginnings. **

**Shinppa-shinaide means "don't worry" and Otōto** **means "little brother".**


	2. My Brother

**My Brother**

Giyuu spent the rest of the afternoon by his brother's bed, gazing at him in soft concern. How could he not realize soon he had lost family somewhere else, it bothered him big time. This boy here, was his younger brother, he'd only known his older sister, Tsutako.

_How come, I wasn't told about my brother? Why was him hidden from me?_

The Water Hashira was getting his mind troubled by thinking much in the situation. He stood in silence, thinking. Lost in his thoughts.

"Tomioka-san?"

_Why didn't Tsutako-Nee-san tell me about our little brother?_

"Tomioka-san?"

_Why... just why..._

"Tomioka-saaaaaaan!"

Giyuu snapped out of it and turned to face a worried Tanjirou. He sighed softly. "What is it, Tanjirou?"

"Etooo-ne... I was wondering how Kyuu-kun was faring." Tanjirou said, poking his fingers together.

"He's been the same, ever since he was brought here. I'm hoping he'll get better soon." Giyuu nodded and looked from Tanjirou back at his little brother.

"I hope so too...did you have another sibling besides Kyuu-kun?"

"...An older sister. Tsutako-Nee-san." Giyuu bit his lip, trying not to show his absolute brokeness over his dear sister's death. He was close to tearing up, really but he would not in front of Tanjirou. He would when he was left alone with his sleeping brother, since no one would watch him breaking.

"What happened to her, Tomioka-san?" Tanjiro asked, approaching and sitting on the chair next to his mentor. Giyuu bit his lip more, he'd tell him the truth.

"When I was younger, before Tsutako-Nee-san got married with her fiance, there was a demon attack and she saved me, sacrificing herself..."

**Flashback...**

Tomioka Tsutako had been getting some details done for her wedding tomorrow, she was smiling darlingly. She was so looking forward to be engaged with the love of her life. Her little brother, Giyuu was currently outside, doing something. When her eyes widened hearing the snarl and break out of the horrible beast.

"Giyuu-!" She darted forward, to go find her brother, when she did so she grabbed his arm and pulled him with her.

"What is going on, Nee-san?!" Giyuu exclaimed with widened eyes as he was dragged. Tsutako pushed him inside one of the rooms of their home.

"Stay here, Giyuu-kun. Don't get out!" She told him firmly.

"B-but, Nee-san-!"

"Now!" Tsutako exclaimed closing the door and going out, with Giyuu yelling after her. The demon had followed her, she tried to strike it, but it went right to her and slashed her back then her chest, she could stare at her brother's horrified face, one last time. "G-Giyuu..." but she in her last breathe slashed off the demon's head, also killing it.

"N-Nee-san?" Giyuu slowly approached when the danger was gone. "Tsutako-Nee-san?" he whimpered and nudged her gently, but with no result. Tears formed in his blue eyes and he burst into tears.

"NEE-SAN!"

That was the last scene Giyuu remembered of that tragic day, it crushed him. Tanjirou could already tell, him asking such touchy question would do this. The Water Hashira was shaking with a hand to his mouth, and as much as he didn't want to show it, the tears flowed.

"Tsu-Tsutako... N-Nee-san..."

Tanjirou panicked and did the first thing that crossed his mind, he quickly embraced the weeping Hashira in a tight hug.

"Gomen'nasai, gomen'nasai, Tomioka-san! Gomen'nasai, I shouldn't have had asked you such touchy question!" Tanjirou franctically apologized while hugging Giyuu close.

Giyuu didn't acknowledge it first, being hugged by Tanjirou. He was so overwhelmed by emotions, he found himself sobbing helplessly at the memory of Tsutako. Only then he realized Tanjirou was hugging him and he lost control of himself, quickly crumbling to his knees, sobbing more louder. Tanjirou, remembered that tragic day where his own siblings and mother were massacred and it got him tears, but he tried not to think about it. He had to comfort his broken Hashira.

"I'm so sorry, Tomioka-san, I'm so sorry. I really should've stayed quiet. Tomioka-san, try to soothe yourself." Giyuu was a sobbing crying mess, it shocked him a lot, he knew him was a firm stone of no emotions. He felt bad for asking such question, knowing it was likely a very touchy subject to Giyuu.

"She died because of me... Sh-she had plans of getting married with the man she loved... I was the cause of her death and her dreams to never come true...I...I..."

"Absolutely wrong, Tomioka-san! You were not the cause of her death, she decided to protect you! You were her number priority back then, she had no choice. But see it this way, she wouldn't have had wanted you to sorrow, she would've wanted you to keep living...I know the feeling, Tomioka-san, I really do." Tanjirou started, holding the Hashira close and patting his back. "My four younger siblings, saved from Nezuko were massacred along with my mother. I knew that my mother's wish was to keep going and protect Nezuko. That was my purpose after they left." He now felt tears of his own softly trickle. "I'm still keeping on, no matter how difficult or hard the road seems... think this as if it was you with Tsutako-san."

Tanjirou's words clicked in Giyuu's mind, his teary eyes widening. These words... why were those so familar? His only friend, Sabito had told him something familiar.

_Your sister was supposed to get married the next day. And despite knowing that she hide you from the demons and protected you. You can't ever die. She left a future to you, Giyuu..._

Giyuu got serious and stopped, he pulled himself together and wiped his eyes. "I'm sorry, Tanjirou. I got overwhelmed by emotions and acted very stupid. I'll keep going for her." He started to get up as the boy helped him.

"Don't say that, Tomioka-san. You're in the right of crying when things get too much. You shouldn't be ashamed of that." Tanjirou smiled, wiping away his own tears. "But that's the spirit, keep going and keep getting strong, that's what your sister would've wanted for you."

"And I shall give her all that. Honor her." Giyuu smiled slightly. "Thank you, Tanjirou." he turned back to his brother, rubbing his forehead. "And I'll do whatever it takes to keep, my little brother safe and living."

Kyuu was sleeping contently, feeling Giyuu's hand, he slowly reached and grasped his wrist, a smile forming in his face, despite still being asleep. Giyuu smiled softly. "I'll protect you, Kyuu. No matter what happens, you're number one in my priorities."

Tanjirou smiled and got up. "Well, I should get going. I'll come check on you later, Tomioka-san." he excused himself and exited the room. Giyuu was grateful of Tanjirou. He was great emotional support. He owned him and he would see to return the boy the favor, if Tanjirou hadn't come, Giyuu wasn't sure how he'd react to all those overwhelming emotions. For now he decided to focus in taking care of his brother.

(The next morning)

Kyuu slowly stirred awake and his eyes slowly opened.

"Slept well?" Giyuu asked softly, there was already a breakfast plate, containing soft rice and some pancakes with berry jam.

"I feel less exhausted, Giyuu-Nii-san." Kyuu nodded and his stomach roared like a demon and the small boy blushed a bit. Giyuu chuckled quietly.

"Good timing, breakfast is ready." he stood and got the tray in front of his brother and set it down. Kyuu sat up and first started with the pancakes as it was expected. Giyuu watched him eat, he already had eating some simmered salmon with daikon earlier, he was just waiting for his brother to wake.

Kyuu finished his pancakes and now started with the soft rice with small bits of simmered salmon. But as soon as he gave it a bit, he suffered an allergic reaction, startling Giyuu entirely.

"Kyuu, what's the matter?!" Giyuu leaned over and took the bowl off the boy's hands, seeing him couth and choke. "KOCHO!" he screamed out urgently. Soon Shinobu had arrived with her assistants.

"What's the matter, Tomioka-san?"

"He ate and then he started coughing like that. Please help him!" Giyuu pleaded, he wasn't about to lose another sibling. Shinobu went straight to action, grabbing a syringe and turning to Giyuu.

"Help me. Hold him down, I need to administer this into his bottom."

At the sight of the needle, Kyuu immediately burst into tears and Giyuu picked him up and placed him down on his stomach. "Hey, hey. It's alright, this is to help you, it'll be okay, I'm here." He soothed, holding him down gently. Shinobu lowered down his hakama pants a bit and while the boy was distracted by his older brother, the Insect Hashira pricked him with the needle. He wailed and Giyuu soothed him, rubbing his head, patting his back, everything he could think off.

Shinobu pulled the needle out and secured it, tossing it in the trash. "There, that shall be instant healing."

Giyuu picked his brother from the bed and huggled him close, he was still hysterical, but the allergy had toned down. The Water Hashira nodded in thanks to Shinobu, while he rocked Kyuu in his arms, trying to soothe him. He knew how awful needles were, when he was he age, he'd gotten many, every year and he had gained a hatred of needles.

"It's okay now, Kyuu. All over." he soothed and Kyuu slowly calmed, not feeling more itch or his throat like it was on fire, he looked more tired than he looked before. Giyuu layed himm down and tucked him in once more. "Lay down and rest more."

The small boy's eyes fought to keep open, they eventually closed and Kyuu fell asleep, his pale face still worried Giyuu a lot. He feared his only family, only discovered a day before to also leave. He wouldn't know what to do if that happened.

"Why is it that everyone in my family must get sick or die?" The Hashira sighed and shook his head, looking down. He could only hope and pray that his younger sibling would go through it.

"Just keep breathing, Kyuu." he placed a hand on his cheek and slightly rubbed it with a sad, tiny smile.

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**I'm up for suggestions of what should happen next! I'd love to hear them out in the reviews. Give my new story some love, New Hope, New Beginnings is the mostly loved. But this one I'm loving it as well.**

**Arigatoo ne!**


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